Chromium electroplating baths are in widespread commercial use for applying protective and decorative platings to metal substrates. For the most part, commercial chromium plating solutions heretofore used employ hexavalent chromium derived from compounds such as chromic acid, for example, as the source of the chromium constituent. Such hexavalent chromium electroplating solutions have long been characterized as having limited covering power and excessive gassing particularly around apertures in the parts being plated which can result in incomplete coverage. Such hexavalent chromium plating solutions are also quite sensitive to current interruptions resulting in so-called "whitewashing" of the deposit.
Because of these and other problems including the relative toxicity of hexavalent chromium, and associated waste disposal problems, extensive work has been conducted in recent years to develop chromium electrolytes incorporating trivalent chromium providing numerous benefits over the hexavalent chromium electrolytes heretofore known. According to the present invention a novel trivalent chromium electrolyte and process for depositing chromium platings has been discovered by which bright chromium deposits are produced having a color equivalent to that obtained from hexavalent chromium baths. The electrolyte and process of the present invention further provides electroplating employing current densities which vary over a wide range without producing the burning associated with deposits plated from hexavalent chromium plating baths; in which the electrolyte composition minimizes or eliminates the evolution of mist or noxious odors during the plating process; the electrolyte and process provides for excellent coverage of the substrate and good throwing power; current interruptions during the electroplating cycle do not adversely affect the chromium deposit enabling parts to be withdrawn from the electrolyte, inspected, and thereafter returned to the bath for continuation of the electroplating cycle; the electrolyte employs low concentrations of chromium thereby reducing the loss of chromium due to drag-out; and waste disposal of the chromium is facilitated in that the trivalent chromium can readily be precipitated from the waste solutions by the addition of alkaline substances to raise the pH to about 8 or above.
The electrolyte of the present invention further incorporates a reducing agent to prevent the formation of detrimental concentrations of hexavalent chromium during bath operation which heretofore has interfered with the efficient electrodeposition of chromium from trivalent chromium plating baths including the reduction in the efficiency and covering power of the bath. In some instances, the buildup of detrimental hexavalent chromium has occurred to the extent that a cessation in electrodeposition of chromium has occurred necessitating a dumping and replacement of the electrolyte. In accordance with a further discovery of the present invention, it has been found that the addition of the reducing agent according to the electrolyte herein disclosed effects a rejuvenation of an electrolyte contaminated with excessive hexavalent chromium restoring the plating efficiency and throwing power of such a bath and avoiding the costly and time consuming step of dumping and replacing the electrolyte.